A thin film transistor having a non-single-crystal germanium film as an active layer (this transistor will be referred to as a non-single-crystal germanium TFT hereinafter) has high mobility and high drivability, and is superior in these characteristics to a thin film transistor having a non-single-crystal silicon film as an active layer (this transistor will be referred to as a non-single-crystal silicon TFT hereinafter). Also, the non-single-crystal germanium TFT can be fabricated at a lower temperature than that for the non-single-crystal silicon TFT. Therefore, it is expected to widen the range of selection of substrates, and realize larger areas and more flexible substrates. For example, thin film transistors having a non-single-crystal germanium film as an active layer are proposed in references 1 and 2.
As described above, the non-single-crystal germanium TFT is superior in many characteristics to the non-single-crystal silicon TFT. However, no non-single-crystal germanium TFT has been put into practical use.
Note that reference 1 discloses a thin film transistor having an oxide film containing aluminum oxide and silicon oxide as a gate insulating layer. Note also that reference 2 does not disclose any practical materials of a gate insulating film.
Reference 1: OPTOELECTRONICS—Device and Technologies, Vol. 1, No. 1, pp. 85–96, Jun., 1986, “TOWARD WALL PANEL TV”, Djamshid Tizabi and Albert George Fischer
Reference 2: Japanese Patent No. 2,855,300
To well achieve the superior characteristics of the non-single-crystal germanium TFT and put it into practical use, it is necessary to reduce the interface state density between an active layer made of non-single-crystal germanium and a gate insulating film. If this interface state density is high, a threshold value variation, leakage current, or the like of the TFT worsens.